New transforming murine type C viruses were characterized with respect to their genome structure and their ability to transform cells in culture: (1) A lung carcinoma-associated virus (LCAV, CI 1-4) was molecularly cloned and shown to have a mink cell focus-forming (MCF)-type structure. Transformed mink lung cells harboring this virus were sensitized to epidermal growth factor (EGF) suggesting that the mechanism of transformation by LCAV may involve sensitization of alveologenic lung cells to EGF. The nucleic acid sequence of CI-3 virus showed that the entire envelope gp70 was derived from an endogenous dualtropic env gene; (2) A new oncogene, raf, from 3,611 virus was molecularly cloned, sequenced, and used for the isolation of its human cellular homologs which were mapped on human chromosomes 3 and 4. The translational products of this virus were P90 and P75 polyproteins which contained viral p15 and p12 structural proteins fused to a tumor gene product lacking tyrosine-specific phosphokinase activity. The two polyproteins differ only by the fact that P90 is a glycosylated form, whereas P75 is myristilated. A suggested link a c-raf to human small cell lung carcinoma and a familial renal carcinoma is being analyzed; and (3) Several differentiating clonal lines were derived from a cell line established from a virus-induced ovarian carcinoma.